This invention relates to a microscope adapted to photomicrography and visual monitoring on a display screen on a selective basis and, more particularly, it relates to a microscope capable of detecting the focal point in an improved way and also to an automatic focusing apparatus.
There are known microscopes adapted to photographing specimens and visually monitoring them on an electronic display screen. Microscopes with such functional features are generally accompanied by a problem of focusing difficulty when the objective lens has a low magnifying power (photomicrographic objective lens). The reasons for this include that the microscope shows a large depth of focus on the specimen side of the objective lens because of a small NA (numerical aperture) value of the objective lens when the lens has a low magnifying power, and an attempt to visually focus on the specimen through the eyepiece can often end up unsuccessfully as the eye adapts itself to the situation to accurately see the object, whereas the microscope shows a small depth of focus on the image side of the objective lens and a slight off-focus condition can become conspicuously recognizable.
Thus, an automatic focusing apparatus for automatically detecting the focal point is considered to be an important accessory for a microscope having a photomicrographic objective lens. Japanese Patent Application Laid-Open No. 61-143710 discloses an automatic focal point detecting apparatus to be used for a microscope that can quickly focus on a specimen having a considerable height.
Meanwhile, the technique of coloring tissues and cells of organisms with fluorescent dye for microscopic observation is known. With this technique, the colored tissues and the cells produce an optical image by rays, but it is impossible to focus on such an optical image produced by infrared rays with a naked eye for the purpose of photographing or displaying on the display screen of an electronic monitor. Therefore, an automatic focal point detecting apparatus is a necessity for such microscopic observation.
Thus, an automatic focal point detecting apparatus to be used for a microscope is typically required to be adapted to visible light and invisible rays such as infrared and/or ultraviolet rays.
However, on the part of such a focal point detecting apparatus, it is accompanied by a problem of chromatic aberration that adversely affects the focusing accuracy of the focal point detecting apparatus, be it for photomicrography or for visual monitoring on an electronic display screen, because of an insufficient color correction capability of the optical system of the apparatus.
FIG. 1 of the accompanying drawings is a graph showing a typical relationship between the wavelength of the optical image and the focal point of a photomicrographic optical system, the vertical and horizontal axes of the graph representing respectively the local point and the wavelength of the optical image of the system. As the graph clearly shows, the focal point can vary remarkably relative to a reference point depending on the chromatic aberration of the optical system.
FIG. 2 of the accompanying drawings is a graph obtained by adding the wavelength vs. focal point curve, or broken curve b, of an automatic focusing (AF) system to the curve a of FIG. 1. The two curves do not agree with each other due to the difference between the optical system of the AF system and that of the photomicrographic system.
The guaranteed focusing range c for both a photomicrographic system and an AF system is defined to be between 460 and 600 nm if the allowable discrepancy between the two focal points is .+-.1.0 mm. In other words, there is no guarantee of focusing under 460 nm or above 600 nm because the difference of the two focal points exceeds .+-.1.0 mm. Differently stated, a photograph taken by using such a photomicrographic system and such an AF system can be blurred if the focal point of the latter is not between 460 and 600 nm.
Thus, the combination of an AF system and a photomicrographic system is inevitably subjected to limitations in terms of wavelength for the above described reason because they use different respective optical systems. Therefore, the AF system may not be relied on in wavelength ranges where an off-focus situation occurs due to chromatic aberration.
It is, therefore, the object of the present invention to provide a microscope and a focal point detecting apparatus that operate with a wide wavelength range of an optical image.